An optical transmitter used in an optical communication system generally includes an optical modulator for modulating a carrier light in accordance with transmission data. An optical modulator configured to include a Mach-Zehnder modulator is known.
FIG. 1 is a diagram illustrating the configuration of a typical Mach-Zehnder modulator. In FIG. 1, an optical waveguide is formed in a surface region of a substrate 100. The substrate 100 has an electro-optical effect. The optical waveguide includes a splitter 101, a pair of waveguides 102 (102a, 102b), and a combiner 103. The splitter 101 splits an input light and directs the resulting light to the pair of waveguides 102a and 102b. Each of the pair of waveguides 102a and 102b propagates the input light. The combiner 103 combines optical signals propagated through the pair of waveguides 102a and 102b. A signal electrode 104 is formed over one of the pair of waveguides 102a and 102b (in the example illustrated in FIG. 1, the pair of waveguide 102a). In the Mach-Zehnder modulator having the above configuration, a data signal is applied to the signal electrode 104. Then, the index of refraction of the pair of waveguides 102 is controlled and a change in the phase difference between the pair of waveguides 102a and 102b is caused by Mach-Zehnder interference. Therefore, an optical signal whose intensity has been modulated in accordance with the data signal is generated.
To increase the amount of data transmission using the optical communication system, transmission schemes that utilize a plurality of Mach-Zehnder modulators have recently been put into practical use. For example, multi-level modulation schemes such as quadrature phase-shift keying (QPSK) (including differential quadrature phase-shift keying (DQPSK)), polarization multiplexing, and other transmission techniques have been proposed. For reduction of the size of an optical transmitter, it is preferable that a plurality of Mach-Zehnder modulators are integrated on a single chip.
FIG. 2 is a diagram illustrating the configuration of an optical device including a plurality of Mach-Zehnder modulators. The optical device includes an input branching waveguide 111 and Mach-Zehnder modulators A and B. The input branching waveguide 111 splits an input continuous-wave (CW) light and directs the resulting light to the Mach-Zehnder modulators A and B. The configuration and operation of the Mach-Zehnder modulators A and B are basically similar to those described with reference to FIG. 1. Note that data signals RF-A and RF-B are applied to the Mach-Zehnder modulators A and B, respectively. Thus, the optical device generates a modulated light A corresponding to the data signal RF-A and a modulated light B corresponding to the data signal RF-B.
An optical device including a plurality of Mach-Zehnder modulators is disclosed in, for example, Japanese Laid-open Patent Publication Nos. 2008-116865 and No. 2007-57785. As a related technology, a Mach-Zehnder modulator having a configuration with improved characteristics without increasing the space between parallel portions of branching waveguides is disclosed in, for example, Japanese Laid-open Paten Publication No. 5-297332.